1. Field of the Invention
The present invention relates to a threading device a sewing machine, and particularly to a threading device for automatically passing a thread through a looper of a sewing machine such as a hemstitch overlock machine a double-loop machine, using pressurized gas.
2. Description of the Background Art
As shown in FIG. 11, a hemstitch over-lock machine is an one-needle/three-thread overlock machine including a needle 31a, which is fixed to a needle rod 30 adapted to move substantially vertically in synchronization with a main shaft and is inclined with respect to a cloth feeding direction. The needle 31a penetrates a needle plate 34 and moves substantially vertically. The one-needle/three-thread overlock machine further includes a lower looper 33, which reciprocates under a needle plate 34 along an arc-shaped path crossing a path of the needle 31, and an upper looper 32, which reciprocates along an elliptical path. This elliptical path crosses the path of the lower looper 33 at the side of the needle plate 34 and crosses the path of the needle 31 above the needle plate 34. The one-needle/three-thread overlock machine operates to form a seam in cloth by crossing a needle thread passed through the needle 31, a lower looper thread S2 passed through the lower looper 33 and an upper looper thread S1 passed through the upper looper 32.
In this overlock machine, as shown in FIG. 12, the upper looper 32 is provided at the vicinity of its tip end with a thread outlet, i.e., thread hole 32a through which the upper looper thread S1, guided by an upper looper thread guide 35, is passed and drawn out. Also, as shown in FIG. 12, the lower looper 33 is provided at the vicinity of its tip end with a thread outlet, i.e., thread hole 33a through which the lower looper thread S2 is drawn out. A thread holding portion 33b is provided with a thread groove 33c so as not to disturb the lower looper thread S2, and a thread guide 33d is provided at the rear portion of the looper. The lower looper thread S2 guided by a lower looper thread guide 36 is retained on a lower looper thread guide 37, and then is retained on the thread guide 33d at the rear portion of the looper before being passed through the thread hole 33b at the tip end. A portion of the lower looper thread S2 between the thread guide 33d at the rear of the looper and the thread hole 33b is accommodated and retained in the thread groove 33c.
In the upper and lower loopers 32 and 33 in the prior art, fine work, using tweezers, was required, for passing the thread through the thread hole 32b in the upper looper and the thread hole 33b in the lower looper, engaging the thread on the thread guide 33d at the rear of the looper, and accommodating the lower looper thread S2 in the thread groove 33c. Further, the thread is passed through the looper part in a complicated manner, as shown in FIG. 12, so that one may make a mistake in the sequence for passing the threads (for example, in the hemstitch overlock machine, the sequence is 1: upper looper thread, 2: lower looper thread, 3: needle thread), resulting in problems such as thread breakage at the beginning of the sewing operation and needle breakage. Since the thread groove 33c provided in the thread holding portion 33b for the lower looper thread S2 is open at the front surface, the accommodated lower looper thread S2 may dangle from the thread groove 33c and become entangled with the other thread, resulting in trouble. Since the loopers 32 and 33 are attached to a narrow portion under the throat plate 34, it is difficult to access them with a fingertip or tweezers, and skill is particularly required for passing the thread through the lower looper thread guide 33d of the lower looper 33 and the thread hole 33a in the lower looper 33 which are located at deep positions. An unskilled person cannot correctly pass the threads, or a long time is required for passing the threads, so that the operation must be interrupted for a long time. During the sewing operation, the entanglement of the threads, which results during a restoring operation, e.g., after the thread breakage, may cause thread breakage again.
In order to overcome these disadvantages, a threader or a threading device has been used (disclosed, e.g., in the Japanese Laid-Open Patent Publication No. 1-303194), or such a method has been employed wherein the looper drive is provided with a clutch, and the threading operation is facilitated by laying or inclining the looper by switching the clutch. However, the threading device has disadvantages such as a complicated structure, unstable operation, and skill required for operation. Also it is difficult for an operator to understand the manner for using the threading device, and thus the sewing machine may be broken due to maloperation.
On the other hand, the applicant has proposed a tubular looper shown in FIG. 2, overcoming the above-noted disadvantages, in which the looper has a hollow structure extending from a thread introduction opening to a tip thread outlet (Japanese Patent Application No. 2-310938). In this looper, the thread does not extend through a complicated thread guide as in the prior art shown in FIG. 12, and the threading operation is complicated only by passing the upper and lower looper threads, which are drawn from thread tensioners, through the tubular loopers via the thread guides 35 and 36.
In the structure employing the tubular loopers, the threading paths are simplified to some extent, and the entanglement and breakage of the threads can be prevented. However, the operation for passing the thread through the tubular looper is a manual operation, so that the threading operation is still troublesome.
Accordingly, it is an object of the invention to provide a threading device of a sewing machine, overcoming the above-noted disadvantages deriving from a thread guide having an extremely complicated structure. Another object of the invention is to provide a threading device of a sewing machine and a threading method, in which a thread can be automatically and quickly passed through a looper, using pressurized gas. Still another object of the invention is to provide a threading device of a sewing machine, in which a thread can be passed selectively through a plurality of loopers. Yet another object of the invention is to provide a threading device of a sewing machine, which can prevent malfunction of a main shaft in a threading operation and malfunction of a threading operation during rotation of the main shaft.